The present invention is particularly directed to vitrified bonded grinding products, e.g. wheels and the like, which are prepared from sol-gel aluminous abrasive grit, a vitreous bond, and a combination of porosity resulting agents which are superior to either alone.
Attempts at generating the requisite total porosity and types of porosity to produce improved vitrified bonded grinding products, especially creepfeed wheels, from sol-gel sintered alumina abrasive grits, especially those having an average particle size greater than about 65 microns, have not been completely successful. There is thus a desire to produce creep-feed and other grinding products containing sol-gel grits in vitrified bonds, which products have an appropriate balance of types of porosity to obtain wheels exhibiting improved grinding performance and excellent makeability.
Sol-gel abrasives are generally made by forming a hydrated alumina gel which may also contain varying amounts of additives such as MgO or ZrO.sub.2 and then drying and sintering the gel. See for example, U.S. Pat. No. 4,314,827. Seeded sol-el abrasives such as those formed by the conversion of hydrated alumina to alpha alumina by the use of alpha alumina seed or functional equivalent materials having the same crystal structure and similar lattice parameters, such as ferric oxide and the like. Processes for the production of seeded sol-gel alumina materials are described, for example, in U.S. Pat. Nos. 4,623,364, 4,744,802, 4,954,462, 4,964,883, and PCT Applications WO 90/08744 and WO 90/02160. Other sol-gel techniques which accomplish a similar result may be used.
To produce abrasive material from a gel, a dried extruded gel material is usually crushed or broken by suitable means such as a hammer or ball mill to form abrasive particles which are generally then size classified. The same type of extrusion, grinding, and forming processes may be used with other abrasive forming compositions than sol-gel materials.
For purposes of this application and the invention disclosed, the term "sol-gel sintered alumina abrasive" is intended to apply to abrasives produced by the teachings of any of U.S. Pat. Nos. 4,518,397, 4,623,364, 4,744,802, 4,954,462, 4,964,883, and PCT Applications WO 90/08744 and WO 90/02160 as well as other sol-gel techniques useful in the art.
Previous attempts at providing porosity in conventional fused white alumina based grinding products include such as:
U.S. Pat. No. 2,085,137 discloses the preparation of abrasive articles by incorporating coke or charcoal in a vitreous bond composition of relatively large abrasive grains, i.e. 40 to 150 grit, to form cellular abrasive structures.
U.S. Pat. No. 2,806,772 discloses in non-vitreous bonded abrasives which are processed at very low temperatures, i.e. below about 260.degree. C., the use of small thin walled balloons of vitrified clay or of heat hardened organic resins to generate porosity in fused alumina abrasive bodies.
U.S. Pat. No. 2,986,455 discloses generally resinoid bonded abrasive bodies containing alumina bubbles in particle sizes as coarse as 6 mesh and finer.
U.S. Pat. No. 3,329,488 discloses organic (resin) bonded abrasive articles containing olivine, a friable pore-forming material which is a mineral of silicates of magnesium and ferrous iron.
Cincinnati Milicron Corp. prepared, at least in 1985, a vitrified bonded grinding wheel which, by examination was found to contain about 86 wt % conventional white fused alumina, about 4.5 wt % bubbled alumina generally having a size greater than about 227 microns, and about 9.5 wt % of a vitrified bond. Microscopic examination also indicated that the product contained some macropores that were believed to have been produced using a conventional pore inducing media that decomposed upon heat treatment, i.e. in drying and/or firing. No sol-gel aluminous grits were found in the product.
When sol-gel alumina vitrified bonded grinding wheels are prepared in accordance with this invention, i.e. containing a combination of both a bubbled alumina and an organic pore inducing material, the resultant wheel is improved substantially over wheels prepared with either material alone and may be operated at higher unit pressure. This results in an improved abrasive grain and overall grinding performance.
Accordingly, it is an object of this invention to produce an improved grinding product from a vitrified bonded sol- gel aluminous grit.
It is a still further object to produce an improved creepfeed grinding wheel.
These and still further objects will be apparent from the following disclosure of the preferred embodiments.